Frequency hopping refers to the transmission of wireless signals by rapidly switching frequencies using a pattern known to both the transmitter and the receiver. Most existing frequency hopping patterns are determined by two encryption keys, including one for the frequency domain and another for time permutation in the time domain. Such frequency hopping is beneficial because it reduces interference and/or noise and, thus, improves the success rate of signal packet transmission when transmitting a signal over a wireless communication network.
Both frequency-hopping and time-permutation keys used in the communications can reduce the probability of detection by malicious interferers or eavesdroppers; however, they cannot reduce the probability of hits in frequency by an interferer. This is because the frequency hopping pattern generated by the existing methods is generally random (i.e., uniformly distributed over the entire spectrum) and independent of channel conditions.
Recently, a dynamic frequency hopping pattern instead of a fixed frequency hopping pattern was adopted in the IEEE 802.22 standard for cognitive radio-based wireless area network (WRAN) applications. A multiple-access (MA) user's signal hops between frequencies sequentially and cyclically in the entire spectrum, and different MA users use different sequence start times to avoid collisions. However, such dynamic frequency hopping patterns are independent of interference conditions, and a user's signal spectrum occupies the entire spectrum. Therefore, the spectrum can cause interference with the same probability as the random frequency hopping pattern by partial band tone interference (PBTI). In addition, the frequency hopping pattern is cyclic, and thus an interferer can detect the frequency hopping pattern of a user with high probability.